(1) Field of the Invention
The present invention relates to the field of display devices. More specifically, the present invention relates to the field of flat panel display devices utilizing liquid crystal display (LCD) technology.
(2) Prior Art
Flat panel displays or liquid crystal displays (LCDs) are popular display devices for conveying information generated by a computer system. The decreased weight and size of a flat panel display greatly increases its versatility over a cathode ray tube (CRT) display. High quality flat panel displays are typically back-lit. That is, a source of illumination is placed behind the LCD layers to facilitate visualization of the resultant image. Flat panel LCD units are used today in many applications including the computer industry where flat panel LCD units are an excellent display choice for lap-top computers and other portable electronic devices. However, because the technology of flat panel LCD units is improving, they are being used more and more in other mainstream applications, such as desktop computers, high-end graphics computers, and as television and other multi-media monitors.
In the field of flat panel LCD unit devices, much like conventional cathode ray tube (CRT) displays, a white pixel is composed of a red, a green and a blue color point or xe2x80x9cspot.xe2x80x9d When each color point of the pixel is illuminated simultaneously, white can be perceived by the viewer at the pixel""s screen position. To produce different colors at the pixel, the intensities (e.g., brightness) to which the red, green and blue points are driven are altered in well known fashions. The separate red, green and blue data that corresponds to the color intensities of a particular pixel is called the pixel""s color data. Color data is often called gray scale data. The degree to which different colors can be achieved by a pixel is referred to as gray scale resolution. Gray scale resolution is directly related to the amount of different intensities to which each red, green and blue point can be driven.
The method of altering the relative color intensities of the color points across a display screen is called white balance adjustment (also referred to as color balance adjustment, color temperature adjustment, white adjustment, or color balancing). In other words, the appearance of xe2x80x9cwhitexe2x80x9d is a combination of red, green and blue intensities in various contributions of each color. xe2x80x9cColor temperaturexe2x80x9d attempts to correlate the temperature of an object with the apparent color of that object. It is the temperature of the light source that illuminates the object. Ideally, that source is a perfect black body emitter, e.g., a thermally radiating object that absorbs all incident radiation and re-radiates that energy with complete efficiency. A theoretical model of such a black body was derived by Max Planck and is the standard to which any the source is compared.
But real life radiators are not so efficient but still tend to follow Planks equation in a relative sense and are known as xe2x80x9cgray bodyxe2x80x9d emitters. A tungsten filament is a very good approximation to a gray body and the user of a tungsten filament as a substitute for a black body reference is wide spread. Therefore, the term xe2x80x9ccolor temperaturexe2x80x9d refers to the emission spectra of a tungsten filament t a given temperature as expressed in degrees Kelvin. In a display, the xe2x80x9ccolor temperaturexe2x80x9d of white correlates to the relative percentage contributions of its red, green and blue intensity components. Relatively high degree K color temperatures represent xe2x80x9cwhitexe2x80x9d having a larger blue contribution (e.g., a xe2x80x9ccoolerxe2x80x9d look). Relatively small degrees K color temperatures represent xe2x80x9cwhitexe2x80x9d having a larger red contribution (e.g., a xe2x80x9cwarmerxe2x80x9d look). Generally, the color temperature of a display screen is adjusted from blue to red while avoiding any yellow-ish or green-ish variations within the CIE chromaticity diagram.
The white balance adjustment for a display is important because many users want the ability to alter the display""s color temperature for a variety of different reasons. For instance, the color temperature might be varied based on a viewer""s personal taste. In other situations, color temperature adjustment may be needed to perform color matching (e.g., from screen-to-screen or from screen-to-paper or screen-to-film). In some situations, color temperature adjustment can correct for the effects of aging in some displays. Therefore, it is important for a flat panel LCD unit to provide the user with a color balancing adjustment option.
One method for correcting or altering the color balance within an LCD unit screen is to alter, on-the-fly, the color data used to render an image on the screen. For instance, instead of sending a particular color point a color value of X, the color value of X is first passed through a function that has a gain and an offset. The output of the function, Y, is then sent to the color point. The function is specifically selected for a particular color temperature result. The values of the above function can be altered as the color temperature needs to be increased or decreased in value. Although offering dynamic color balance adjustment, this prior art mechanism for altering the color balance is disadvantageous because it requires relatively complex circuitry for altering a very large volume of color data. The circuitry adds to the overall cost of production and can increase image generation latency. Secondly, this prior art mechanism may degrade the quality of the image by reducing, e.g., narrowing, the gray-scale range and therefore the gray-scale resolution of the flat panel display. Therefore, it is desirable to provide a color balance adjustment mechanism for a flat panel display screen that does not alter the image data nor compromise the gray-scale resolution of the image.
Another method of correcting for color balance within a flat panel display screen is used in active matrix flat panel display screens (AMLCD). This method pertains to altering the physical color filters used to generate the red, green and blue color points. By altering the color the filters, the color temperature of the AMLCD screen can be adjusted. However, this adjustment is not dynamic because the color filters need to be physically (e.g., manually) replaced each time adjustment is required. Therefore, it would be advantageous to provide a color balancing mechanism for a flat panel display screen that can respond, dynamically, to required changes in the color temperature of the display.
Within CRT devices, color balancing is performed by independently altering the voltages of the primary electron guns (e.g., red, green and blue guns) depending on the color temperature desired. However, like the prior art mechanism that alters the color data on-the-fly, this prior art color balancing technique reduces the gray-scale""s dynamic range and therefore the gray-scale resolution of the display. Also, this technique for color balancing is not relevant for flat panel LCD units because they do not have primary electron guns.
Accordingly, the present invention offers a mechanism and method for providing color balancing within a display that does not require a large amount of complex circuitry and does not reduce the gray-scale resolution of the display. Further, the present invention offers a mechanism and method that dynamically alters the color balance of a display and is particularly well suited for application with flat panel LCD units. These and other advantages of the present invention not specifically described above will become clear within discussions of the present invention herein.
Multiple light source systems are described herein for color balancing within a liquid crystal flat panel display unit. The present invention includes a method and system for altering the brightness of two or more light sources, having differing color temperatures, thereby providing color balancing of a liquid crystal display (LCD) unit within a given color temperature range. The embodiments operate for both edge and backlighting systems. In one embodiment, two planar light pipes are positioned, a first over a second, with an air gap between. The light pipes distribute light uniformly and independently of each other. The first light pipe is optically coupled along one edge to receive light from a first light source having an overall color temperature above the predetermined range (e.g., the xe2x80x9cbluexe2x80x9d light) and the second light pipe is optically coupled along one edge to receive light from a second light source having an overall color temperature below the predetermined range (e.g., the xe2x80x9credxe2x80x9d light).
In the above embodiment, the color temperatures of the first and second light sources are selected such that the overall color temperature of the LCD can vary within the predetermined range by altering the driving voltages of the first and second light sources. In effect, the LCD color temperature is altered by selectively dimming the brightness of one or the other of the light sources so that the overall contribution matches the desired LCD color temperature. In the selection of the light sources, a constraint is maintained that at any color temperature the brightness of the LCD is not reduced below a given threshold minimum (e.g., 70 percent of the maximum brightness). In the selection of the light sources, a second constraint is maintained that within the predetermined color temperature range, the color temperature is held close to the black body curve of the CIE chromaticity diagram. In a third constraint, the light sources are selected so that their maximum brightness point is set to be near the middle of the predetermined color temperature range.
In furtherance of one embodiment of the present invention, the light sources selection process may be implemented in computer readable instructions executable by a computer system and stored in computer readable memory such that a large number of number of light sources candidates may be simulated to obtain their luminance, chromaticity, and color temperature data. Candidates that satisfy the above constraints are selected. In one embodiment, the selection process includes the step of analyzing high-resolution spectral files of R, G, and B phosphors, varying a percentage composition of the R, G, B phosphors to generate multiple sets of light source candidates, matching up the light source candidates to generate a pool of candidate pairs, calculating a color temperature-luminance relationship for each candidate pair, and rejecting the candidate pair unless the color temperature-luminance relationship satisfies the above predefined selection constraints.
In another embodiment of the present invention, the selection process specifically includes the steps of calculating a chromaticity relationship for each candidate pair and rejecting the candidate pair if the chromaticity relationship deviates significantly from the black body curve. In yet another embodiment of the present invention, the selection process further includes the steps of examining the color temperature-luminance relationship to determine whether a peak brightness point occurs at the middle of the given color temperature range.
One embodiment of the present invention includes a color balancing system within a flat panel display for providing color balancing within a color temperature range, the color balancing system having: a first planar light pipe disposed to provide backlight to a liquid crystal display (LCD) layer; a first light source optically coupled to provide light to the first planar light pipe, the first light source having a color temperature that is below the minimum color temperature of the color temperature range; a second planar light pipe disposed parallel to the first planar light pipe such that an air gap exists between the first and the second planar light pipes, the second planar light pipe also for providing backlight to the LCD layer; a second light source optically coupled to provide light to the second planar light pipe, the second light source having a color temperature that is above the maximum color temperature of the color temperature range; a pre-polarizing film disposed between the light pipes and a rear polarizer of an LCD; and a rear reflector positioned on the other side of the light pipes. The system also has a circuit coupled to the first and the second light sources for setting a color temperature of the flat panel display by selectively and independently varying the brightness of the first light source and the brightness of the second light source. The circuit decreases the brightness of the first light source to increase the color temperature of the flat panel display and decreases the brightness of the second light source to decrease the color temperature of the flat panel display.
Significantly, in the present embodiment, brightness in the LCD is enhanced by polarization recycling. According to the present embodiment, the pre-polarizing film first pre-polarizes light emitted from the light pipes to a predetermined orientation that matches the polarization orientation of the rear polarizer of an LCD. Light that is not polarized in the predetermined orientation is reflected by the pre-polarizing film to the reflector where it is rephased to the predetermined orientation. Consequently, brightness of the LCD screen is significantly improved by the recycling. In one embodiment, the pre-polarizing film comprises a layer of DBEF brightness enhancement film, and the rear reflector is made of a PTFF material. In another embodiment, the rear reflector is covered with a film comprising barium sulfate. Brightness may also be significantly enhanced by the addition of a crossed BEF layer between the rear polarizer of the LCD and the light pipes.